This invention relates to an improved apparatus for exposing photosensitive media. More specifically, this invention relates to such an apparatus which is selectively operable to compensate for variations in light required to expose photosensitive media. Such variations in light required for exposure appear to arise from factors such as variations in density (transmissivity) of originals through which the photosensitive media is exposed; variation in the transmissivity of photosensitive media when light passing through the media is sensed; and variations in the response of the photosensitive material to light passing through originals of varying density. The density variations in originals can result from, for example, varying the number of originals through which the photosensitive media is exposed and variations in the density of individual original sheets.
Known print-making devices include a flat glass plate with a light source on one side and a lid or curtain on the other side. One or more original sheets or films to be copied are positioned on the glass plate above the light source and an overlying sheet of photosensitive material (i.e. diazo) is placed on top of the originals. The light source is turned on to expose a photosensitive layer of the photosensitive material.
In the print-making device disclosed in U.S. Pat. No. 4,526,463 of Hickey, et. al., a photosensor receives light passing through one or more original sheets to the photosensitive media. An integrator circuit integrates and thereby monitors the total light received by the photosensor over time. This circuit produces an integrator output voltage signal corresponding to the received light. A comparator circuit compares this integrator output signal with a reference voltage signal established by an operator of the print-making apparatus. When the desired amount of light has been received by the photosensor, the integrator output signal matches the reference voltage signal, and a correct exposure is deemed to have occurred. When this happens, the supply of light to the photosensitive media is shut off. The photosensitive media is then developed in a conventional manner. Because light passing through the original sheets to the sensitized media is monitored, this previously patented apparatus compensates somewhat for factors such as differences in density of the original sheet or sheets and variations in the intensity of light from the light source.
As another aspect of the U.S. Pat. No. 4,526,463 print-making apparatus, a "base tone" control operates to vary the relative capacitance which is coupled to the integrator circuit during first and second partial exposures. This facilitates multiple partial exposures of the same photosensitive media. With this technique, for example, portions of the resulting print will be faded relative to other portions of the print. Also, in such a print-making apparatus which was sold more than one year ago, in addition to the "base tone" control, a selection switch was provided for varying the magnitude of the capacitance included in the integrator circuit between two levels. By varying the capacitance, the range over which the device of U.S. Pat. No. 4,526,463 could be conveniently operated was enhanced.
Although the print-making device described in the prior Hickey et. al. patent greatly facilitates print-making and improves the quality of resulting prints in many circumstances, it does suffer from several drawbacks. In particular, when exposing photosensitive media through varying numbers or density of originals, in some cases the photosensitive media is incorrectly exposed to a substantial degree. Although this could be corrected to a certain extent by adjusting the reference voltage depending upon the number and density of originals, it is desirable to minimize the number of adjustments that need to be made as prints are produced. This incorrect exposure was more pronounced when the light sensor of the prior patented device was positioned to sense light passing through both originals and the photosensitive media in comparison to the case when light was sensed at the photosensitive layer.
Therefore, an improved apparatus for exposing photosensitive media is desirable to overcome these and other problems of prior art devices.